The present invention relates to a micro-motor, particularly to a micro-motor that presents low cogging torque in operation.
Cogging torque in permanent magnet motors is produced by the magnetic interaction attraction between the permanent magnets and the stator""s salient teeth when the current is not provided. Because the direction of the cogging torque is opposite to that of the running torque produced after the current is supplied, the running torque must be counterbalanced a little by the cogging torque such that the motor can be deiven to rotate. In the operation of motors, high cogging torque acts as an obstacle for motor""s start-up rotation, a cause of vibration, and a source of noise. It may consequently degrade the motor""s life and controllability.
To solve the problem, design goals in prior technology generally focus on optimizing the magnet""s magnetization or finding an optimum shape for the stator. In the methods for optimum shape, complex formulas are required. As shown in FIGS. 1(a) and 1(b), U.S. Pat. No. 6,044,737 provides an optimization of the profile of the stator made of silicon steel. In the figure, a salient pole on the stator is profiled along the dash line linking the points P, C, and G. The profile is determined by specific formulas.
In the diametrically wound and diametrical air gap type of brushless motor, as shown in FIG. 1(a), the relation between an angle A of the central arc and a radius r of the modified arc follows the formula:             5      33        ⁢          (                                    360            ∘                    N                -        b0            )        ≤  A  ≤            1      3        ⁢          (                                    360            ∘                    N                -        b0            )      
Where A is half the angle subtended by the central arc, N indicates the number of salient poles, and b0 indicates the angle subtended by the groove opening. Furthermore, as is shown in FIG. 1(b), the radius R and the radius r are preferably within the limits set by the following formula:                               2          ⁢                      R            ⁢                          (                              R                -                t                            )                                ⁢                      (                          1              -                              cos                ⁢                                  xe2x80x83                                ⁢                θ                                      )                          +                  t          2                            2        ⁡                  [                      R            -                                          (                                  R                  -                  t                                )                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              θ                                ]                      ≤    r    ≤                            2          ⁢                      R            ⁢                          (                              R                -                                  t                  4                                            )                                ⁢                      (                          1              -                              cos                ⁢                                  xe2x80x83                                ⁢                θ                                      )                          +                              (                          t              2                        )                    2                            2        ⁡                  [                      R            -                                          (                                  R                  -                                      t                    4                                                  )                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              θ                                ]                          θ    =                            180          ∘                N            -              b0        2            -      A      
Where, R indicates the radius of the central arc and t indicates the thickness at the edge of the teeth section.
In another kind of prior technology, a xe2x80x9cslot-pole ratioxe2x80x9d, which indicates the ratio of a slot angle to a pitch angle, is optimized to decrease the cogging torque. As shown in FIG. 1(c), Taiwan Patent No. 404621 proposes an axially coiled air gap type of motor including a silicon steel sheet 41 that comprises a plurality of salient poles 43. Each of the salient poles 43 has a symmetrical profile, such as the arc shown in the figure. An aperture 44 is formed between each of the adjacent pole pairs. In this patent shown in FIG. 1(c), to optimize the profile, the ratio of aperture angle (B-A) to pitch angle (indicated by B) is preferably between 0.55 and 0.68.
These prior arts, however, have many disadvantages.
For example, the formulas for the stator""s profile are rather complex, and this profiling method is not adequate for the manufacture of micro-motors. Thus, it is preferable to propose an innovative design to solve the cogging torque problem.
The present invention proposes a micro-motor with low cogging torque. The configuration of the micro-motor according to the present invention is much simpler than that of the prior technology. Furthermore, in the assembly of the present invention, conventionally used silicon steel sheets may satisfy the functional requirement of the micro-motor thereby greatly reducing the manufacturing time and cost. A new profile of the silicon steel sheet, which relates to cost increase, is therefore not necessary.
The micro-motor of the present invention is comprised of a stator and a rotor. The stator is comprised of a plurality of magnetically conductive sheets stacked together. The magnetically conductive sheets are each formed with a plurality of a symmetrical salient teeth. Furthermore, in the magnetically conductive stack, one of the magnetically conductive sheets is disposed reversely relative to the other magnetically conductive sheets.